1. Field of the Invention
This invention relates to digital communication systems and in particular to a method and apparatus for detecting data corruption in such systems. The invention finds particular, though not exclusive, application in the field of cordless telephony, in particular the new CT2 digital systems.
2. Description of the Related Art
In such CT2 digital systems, imperfections in the transmission system may cause the data to be corrupted, e.g. due to interference or fading on an RF link. In many such applications, it is possible to transmit extra information to permit error detection and/or correction at the receiving end. Such methods include parity checks, cyclic redundancy codes, etc. In other applications however, the increase in data rate or transmission time which is needed in order to append such information is not practical.
The present invention relates to such situations and provides a method and apparatus for error rate detection in digital communication systems, particularly those having limited bandwidth. The invention permits monitoring of data integrity without requiring any extra data to be transmitted. It has the advantage that the onset of corruption of the data can often be anticipated, i.e. detection of link degradation extends into the region where data can still be correctly recovered, as well as situations where the data is actually corrupted.
The specific application in which the invention is intended to be used is in the transmission of digitized speech over an RF link in a CT2 digital cordless telephone system. Here, not enough bandwidth is available to add error detection/correction data to the speech data. Therefore, it is necessary to detect poor reception conditions due to low signal strength or interference in order to prevent the user being subjected to the loud noises that the speech decoder generates in the presence of significant bit error rates.
Monitoring of the received RF signal strength is commonly used to predict the onset of corruption in radio applications; however, this technique cannot detect corruption by an interfering signal as the signal strength of the principal signal may well remain high. Also, the correlation between signal strength and data degradation varies between RF units so such predictions may be inaccurate.
When noise or interference is added to a bandlimited data stream, two effects occur. Firstly the recovered value of the bit may be corrupted. This corruption is usually not detectable if no error detection data is sent--the transmitted data could have any value. Secondly the time of the transitions between bit values will be altered from their normal position (jitter). The invention detects such jitter and uses it as an indication of (possible) bit corruption.
Already known is a synchronizing circuit which takes a number of samples of an incoming signal per bit and uses some function of the values of these samples (e.g. a majority vote) to determine the correct value of the received data bit. Methods of automatically gaining synchronism using the value of the samples are also known.
The technique of the present invention involves taking a number of samples per incoming bit, and also using some function of the values of these samples to determine whether the jitter present exceeds one or more thresholds. We may then either give a single indication of the presence and/or degree of such jitter, or periodically give an indication of the number and/or degree of such occurrences. These indications are used to assess the quality of the communications link and detect degradation, enabling some corrective or preventative action to be taken.